Thermionic cathodes for mhd generators

ABSTRACT

Efficiency of non-equilibrium magnetohydrodynamic generators is markedly increased by provision of thermionically emitting cathodes. Cesium in concentrations 0.1 to 0.3 percent improves plasma conductivity, and when deposited on hot tungsten vastly improves thermionic emission but only in absolute concentrations which, near atmospheric pressure, greatly exceed 0.1 to 0.3 percent. Cesium is applied topically in sufficient concentrations to tungsten surface either from dispenser arrangement in which cesium passes through tungsten cathode face, or through apertures in channel wall adjacent to tungsten cathode face; amounts used are such that, after passage into main gas stream and mixture therewith, cesium concentration is still within permissible limits for improving plasma conductivity. Alternatively, generator is operated at high gas pressure (e.g., 5 atmospheres) so that the requisite absolute cesium concentration for thermionic emission is only the permissible 0.1 to 0.3 percent of the concentration of the pressurized plasma. The Invention herein described was made in the course of or under a contract or subcontract thereunder, (or grant) with the Department of the Navy.

FIFSSUZ 3R Zauderer 1 Jan. 2, 1973 54] THERMIONIC CATHODES FOR MHDGENERATORS [75] Inventor: Bart Zauderer, Bala Cynwyd, Pa. [73] Assignee:General Electric Company [22] Filed: Sept. 9, 1971 [21] Appl. No.:178,881

3,355,605 11/1967 Okress Primary Examiner-D. X. Sliney Att0rneyAllen'E.Amgott et al.

[5 7] ABSTRACT Efficiency of non-equilibrium magnetohydrodynamicgenerators is markedly increased by provision of thermionically emittingcathodes. Cesium in concentrations 0.1 to 0.3 percent improves plasmaconductivity, and when deposited on hot tungsten vastly improvesthermionic emission but only in absolute concentrations which, nearatmospheric pressure, greatly exceed 0.1 to 0.3 percent. Cesium isapplied topically in sufficient concentrations to tungsten surfaceeither from dispenser arrangement in which cesium passes throughtungsten cathode face, or through apertures in channel wall adjacent totungsten cathode face; amounts used are such that, after passage intomain gas stream and mixture therewith, cesium concentration is stillwithin permissible limits for improving plasma conductivity.Alternatively, generator is operated at high gas pressure (e.g., 5atmospheres) so that the requisite absolute cesium concentration forthermionic emission is only the permissible 0.1 to 0.3 percent of theconcentration of the pressurized plasma. The Invention herein describedwas made in the course of or under a contract or subcontract thereunder,(or grant) with the Department of the Navy.

7 Claims, 6 Drawing Figures 30 PUMP 3.?

655/044 M /5 ii 20" I I l \\L\\\\\\\\\\ N05L (5A5 PfiE/ON/ZER :24 c0020?[I] PR5$UR 3 HEAT L SOURCE 32 COMP/M350 l /0 Jo I 34 THERMIONIC CATHODESFOR MHD GENERATORS BACKGROUND OF THE INVENTION 1. Field of the InventionThis invention pertains to magnetohydrodynamic generators.

2. Description of the Prior Art confined to subsonic plasma flows,because stagnation pressures of 100 atmospheres are required for super-The addition of cesium vapor as a seeding material to 10 SUMMARY OF THEINVENTION It has been found that the electrode voltage loss which issubtracted from the induced voltage, and appears as a reduction ofoutput voltage, is of the order of 100 volts in conventional MHDgenerators, which do not have high-density thermionic cathodes. It hasbeen reported (Advances in Electronics and Electron Physics, L. MartonEditor, Academic Press, New York City, 1962, page 147) that puretungsten electrodes heated between l,200 and 1,500 K. will emit up to 50amperes per square centimeter in a cesium-seeded noble gas atmosphere ifthe arrival rate at the tungsten surface is greater than 10 cesium atomsper square centimeter per second. Unfortunately, it has been found thatin non-equilibrium MHD generators it is desirable to have a very lowcesium proportion, e.g., 0.1 to 0.3 percent of the total atoms presentfor two reasons. First, the elastic electron-cesium cross section isvery large compared to the electron-noble gas cross section, whichcauses a large cesium concentration to reduce the conductivity of theplasma, which is inversely proportional to the elastic cross section.Secondly, a non-equilibrium plasma in a magnetic field is susceptible toan ionization instability which sharply reduces the effective plasmaconductivity. Experimental evidence (Vitshas, A. F. et al., Electricityfrom MHD, Vol. 1, page 29, International Atomic Energy Agency, Vienna,1968) indicates that if the cesium is fully ionized, which implies a lowcesium concentration of less than 0.2 percent, the effect of theionization instability on the conductivity is markedly reduced. But foreffective thermionic emission the cesium density, regardless of thetotal gas pressure, must be a minimum of about 10 per cubic centimeter.Now, dividing Avogadros number by the gram molecular volume gives adensity of about 2.7 X lmolecules per cubic centimeter at N.T.P.; and ate.g., l,900 K and 1 atmosphere pressure the density will be about 4 X 10molecules per cubic centimeter. Thus the minimum cesium concentrationfor thermionic emission will be about 2.5 percent of the total muchhigher than the concentration desirable for good plasma conductivity. Ihave found two basic ways of providing cesium concentration adequate forthermionic emission, while maintaining a cesium concentration in theactive volume of plasma which is compatible with good plasmaconductivity.

The first method is to operate the noble gas atmosphere well aboveatmospheric pressure, atmospheres or higher, which will raise the totalgas density to about 2 X 10 or more, making the cesium concentration of10 only 0.5 percent of the total. This is sonic operation at Mach 1.5 to2.5, and at such high pressures the conductivity of helium is too low toproduce non-equilibrium ionization even at magnetic fields as high as10' gauss. Argon can harbor nonequilibrium ionization at such pressures,but even full ionization of the cesium would still result in a highcathode sheath drop.

The second method, which has several possible embodiments for execution,is to provide a source of high density of cesium vapor adjacent to thethermionic cathode surface and allow the cesium vapor thus introduced tomix with the working gas so that its concentration in the plasma ingeneral is within the permissible limits for high plasma conductivity.This I do by feeding cesium through a porous tungsten plug, or,alternatively, by feeding cesium vapor through apertures in the channelwall adjacent to the heated tungsten cathode surface so that asufficiently high concentration of cesium vapor will be provided to thetungsten surface, but the cesium concentration will fall elsewherethrough the working gas to a suitably low value. Basically, I propose anon-equilibrium distribution of cesium vapor to provide differentconcentrations as required at different locations. This method, becauseit affords flexibility to meet different conditions, is a convenientmanner of practicing my invention. The choice between the use of aporous dispensertype cathode and provision of cesium vapor throughseparate apertures is best determined by design considerations.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 represents schematically andpartly sectioned an MHD generator.

FIGS. 2, 3, 4, 5, and 6 represent various cathode structures suitablefor practicing my invention in a generator as represented in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 represents generallyschematically, and in section, MHD apparatus of the kind to which myinvention is applicable. A heat source 10 is connected thermally to astore 12 of noble gas, under pressure, which is seeded by cesium seederl3 with cesium to promote ionization. These are purely conventional, andrepresented by simple rectangles. The seeded noble gas passes through aduct 14, shown in section and may pass through various preionizing means16, although this is optional, and into an MHD channel 18, also shown insection. As is well known in the art, the MHD channel must be insulatingand refractory to as high a temperature as possible. It is thereforeusually made of ceramic material best meeting these requirements. It ishornbook doctrine in MHD that materials are not available which willwithstand the temperature at which gases become highly conductive fromthermal ionization; and therefore it is necessary to employ lowertemperatures, e.g., 2,000 K, at which materials can survive, and toproduce conductivity in the gas by a variety of stratagems. Seeding witha material such as cesium is a practical necessity in order to promoteat least an initial ionization in the gas in order that the variousother methods for increasing the ionization may function effectively.But since some cathodes according to the teachings of my invention willadd some cesium to the already seeded gas, the proportion of cesiuminitially seeded by cesium seeder 13 may be toward the low side of thedesirable 0.1 to 0.3 percent range. Pole pieces and 22 of a magnetproduce a magnetic field transverse to the direction of gas flow,parallel to the plane of FIG. 1. Anodes 24 are mounted in the back wallof the channel; while these are represented as subdivided only in thedirection of gas flow, there are some advantages to subdividing themalso at right angles to the direction of gas flow that is, the longstrips shown may be cut horizontally (in FIG. 1) to form a larger numberof smaller electrodes. These are all of a suitably refractory metal,must all have separate connections extending backward through thechannel wall, and are preferably designed to be relatively cool since itis desirable that, being anodes, they not emit electrons. An exit tube26 for discharge of the gas is also represented. Exit tube 26 leads to acooler-filter 28 where the cesium vapor is condensed to a liquid, thecesium droplets (and any accidental particulate matter in the gas) beingfiltered out. The liquid cesium is returned to cesium seeder 13 viaconduit 30, which is also provided with a branch outlet 32 which will beused only for embodiments (such as those represented in FIGS. 3, 4, 5,and 6) which require to be supplied with cesium. The coolerfilter unitis conventional in the art; and a pump 33 is provided for returning theliquid cesium under suitable pressure to permit its being fed to thevarious stores where it is required. A compressor 34 receives the cooledand filtered gas and returns it to the store 12 of noble gas underpressure. Thus the system is completely closed, except for heat, theworking materials being recirculated.

FIG. 2 represents a cathode suitable for use in a first embodiment of myinvention in which the cesium present in the gas is only thatcontributed by the seeder l3, and the pressure of the noble gas is madesufficiently high that the absolute concentration of cesium which isnecessary for good thermionic emission is only the 0.1 to 0.3 percentwhich is desired for high and stable conductivity. A tungsten plug 40 isrepresented inserted through the wall of channel 18, this wall being theone which does not appear in FIG. 1 because channel 18 is thererepresented sectioned so that the wall which would be nearer to theviewer of FIG. 1 has been removed by the sectioning. It is this latterwall which is represented in FIGS. 2, 3, 4, 5, and 6. The part oftungsten plug 40 which is external to channel 18 is represented asprovided with heater elements 42 to indicate the provision of means foradjusting the temperature of plug 40 so that its upper surface will havea temperature suitable for thermionic emission. However, it must berecognized that the channel 18 will contain gas at a high temperatureand therefore it is possible that it will be necessary (depending uponthe thermal flow characteristics of the particular apparatus) to coolplug 40 to adjust its temperature to the desired value. Thus heaterelements 42 should be regarded as representing temperature adjustingmeans which may heat or cool, rather than exclusively as heating means,even though the conventional resistor symbol has been used forsimplicity of illustration. The shape of plug 40 in the cross sectionwhich does not appear in FIG. 2, since it would be normal to the paper,depends upon the electrode pattern chose; in general, for each anode 24,there should be opposed to it a cathode surface of approximately thesame shape. The relation between cathode and anode is one to one, witheach electrode of either kind having its separate connection external tochannel 18.

In this embodiment the entire system operates at a pressure such thatthe requisite concentration of cesium to promote adequate electronemission from cathode plugs 40 is introduced by cesium seeder 13; butthe pressure in the system is so high that this concentrationconstitutes only the desirable 0.1 to 0.3 percent concentration. Thepressure required is about 5 atmospheres. The reason this tour de forceis actually operative is that the cesium supply required by the cathodesis an absolute value, substantially indepen dent of pressure; but theconcentration limit desirable for good plasma conductivity is relative,that is, a fraction of the total gas concentration. Hence what one doesin practicing this form of the invention is to alter the total gasconcentration so that the absolute concentration required for thermionicemission promotion becomes only the fraction desirable for high plasmaconductivity. While the apparatus for operation under pressures of 5atmospheres is naturally heavier and somewhat more complex than that foroperation at lower pressures, the size of generator for a given outputis somewhat reduced, so that this method merits consideration dependingupon the operating requirements.

The remaining figures are embodiments of various cathode electrodearrangements in execution of the second basic form of my invention, inwhich the concentration of cesium vapor adjacent to the thermionicallyemissive face of the tungsten is adjusted to a desirable valueindependently of the concentration of cesium provided by cesium seeder13.

In FIG. 3, a plug 44 of porous tungsten is inserted through the wall ofchannel 18, as solid plug 40 is inserted in FIG. 2. The lower end ofplug 44 is fitted into the upper end of a cylinder 46, in whose lowerend (in FIG. 3) a piston 48 moves. The rod of piston 48 has a collar 50against which a spring 52 bears, tending to push piston 48 into cylinder46, the opposite end of spring 52 bearing against a stop 54. Cylinder 46is filled with a charge of cesium 56 which is kept molten, heaterelements 58 being represented as surrounding the cylinder 46 to indicatethe possible need to supply heat for this purpose. The part of plug 44which lies on the side of the wall of channel 18 which outside of thechannel is represented as provided with heater elements 42, as in FIG.2. However, it must be recognized that the channel 18 will contain gasat a high temperature and it is possible that it will be necessary(depending upon the thermal flow characteristics of the particularapparatus) to cool plug 44 and, possibly, even cylinder 46 to adjust itstemperature to the desired value. Thus heater elements 58, like 42,should be regarded as representing temperature adjusting means which mayheat or cool, rather than exclusively as heating means, even through theconventional resistor symbol has been used for simplicity ofillustration. The shape of plug 44 in the cross section which does notappear in FIG. 3, since it would be normal to the paper, depends uponthe electrode pattern chosen, just as for plug 40 of FIG. 2. Inoperation of the embodiment represented in FIG. 3, the cesium 56 is keptmolten, preferably at 500 to 700 K, and plug 44 is kept so that itsupper surface is at a temperature preferably in the range 1,300 to 1,400K. The molten cesium 56 cannot flow directly through the capillary poresin plug 44, but it vaporizes at its contact with plug 44 and the vaporflows through the pores of plug 44 and appears at the upper face of plug44 where it promotes thermionic emission from the hot face, from whichit evaporates but is constantly replaced by flow of further cesium vaporthrough plug 44 from the molten charge 56. The concentration of cesiumvapor immediately adjacent to the upper face of plug 44 will in generalbe above the maximum desirable for good plasma conductivity; but becausethe surfaces of all the cathodes are only a fraction of the totalchannel surface, the total cesium vapor introduced from the cathodesurfaces will not raise the total cesium concentration in the flowingplasma above the desirable maximum of about 0.3 percent. Thus this andsucceeding embodiments provide cathodes having a cesium supplysufficient for high density of electrons emitted without raising thetotal cesium concentration in the plasma impermissibly. While it has notbeen represented, to avoid complication of the drawing, conduit branch32 of FIG. 1 may in practice be connected to return condensed cesium tothe interior of cylinder 46. Indeed, it would be possible to replacecylinder 46 by a closed chamber fed cesium at the desired pressure, andeliminate piston 48 and spring 52 which serve merely to produce thedesired pressure. However, in developmental work, the spring and pistonoffer the advantage of ready adjustment of the cesium pressure byadjustment of the force produced by spring 52.

FIG. 4 represents a modification of the embodiment FIG. 3 in that cesium56 is maintained in a boiler 60 where cesium is actually vaporized andpasses into a housing 62 which conducts the vapor to plug 28, throughwhich it flows with effects as described for FIG. 3. It has possibleadvantages of slightly more simple mechanical execution in not requiringa moving piston; and it would be possible to design it so that heat flowfrom plug 44 to cesium 56 would be negligible, making the rate of cesiumvapor flow more completely dependent upon the rate of heat supply byheater elements 58, and thus more readily adjustable. As with FIG. 3,cesium from conduit branch 32 may be fed back to boiler 60.

FIG. 5 represents a still further modification of the embodiment of FIG.3 in which a tungsten plug 64 is provided with tubular holes 66 throughwhich cesium vapor flows from liquid cesium 56 in a boiler 68 which alsoserves as a housing to lead the vapor to the openings of holes 66. Thisembodiment may be preferable if there is any difficulty in a giveninstallation with solid contaminants depositing upon porous plugs 44 andclogging their pores. The general mode of operation of this embodimentis similar to that of the embodiments previously described.

FIG. 6 represents a further mechanical modification of the structure ofcathodes operating according to my basic teachings. Here wall 18 of thechamber is pierced with slots 70 located upstream (as indicated by thearrow showing the direction of plasma flow) of solid tungsten plugs 72.Cesium vapor, produced from a store of molten cesium 56in boiler 74,passes through slots and is blown by the moving plasma across theemissive face of plug 72, promoting thermionic emission in the mannerpreviously described. It should be observed that this embodiment lendsitself particularly well to the use of a single cesium boiler connectedto a number of slots feeding a number of cathodes, and thus permittingsome simplication in apparatus design. Cesium from conduit branch 32 maybe fed back to such boilers as 68 and 74.

It is evident that the various embodiments I have shown, or othersembodying the same principles but modified within the skill of the art,may each, in specific circumstances, be the one to be preferred for aparticular apparatus design.

The net virtue of any of these embodiments of my invention is that theypermit efficient continuous operation of MI-ID generators by permittingone to obtain simultaneously the high plasma conductivity which isrequisite to high efficiency and high output and the lowered electrodedrop which is also necessary to high efficiency.

Other particular known embodiments of the various components of theembodiments of MHD generators that I have disclosed may also be employedin the practice of my invention. For example, in Electricity from MHD,"International Atomic Energy Agency, Vienna, I968, Volume I, Pages397-408, a paper by Barouezec and Salvat discloses thermionic cathodesshaped like circular cylinders, provided with internal ducts forbringing cesium to the cathode surface, and suspended inside the MHDduct so that it is subject to free flow of plasma over almost its entiresurface, except for very small supports. The object of interest to theseauthors was avoidance of the extreme concentration of current which theHall effect produces in the edge of substantially orthogonally boundedelectrodes, such as metal strips. However, there is no reason why such acathode should not be applied in the practice of my invention.

Generalizing the description of the embodiments, and employing referencenumbers from the drawings, 12 is a source of noble gas under pressure,10 is means for heating the noble gas, 13 is means for providing cesiumvapor in the noble gas, 18 is a channel of refractory insulatingmaterial which receives the gas moving as a result of its initialpressure and guides it normal to the magnetic field produced betweenpole pieces 20 and 22, and between pluralities of anode electrodes 24and cathodes of the various forms shown in FIGS. 2 through 6.

The various containers for cesium S6 and the vapor passages associatedwith them are cathode cesium means. Tungsten provided with interiorpassages is represented by 44 and 64, those of 44 being sufficientlysmall to stop the passage of liquid molten cesium, which in FIG. 3 is incontact with the tungsten, and in FIGS. 4 and 5 is apart from thetungsten so that only its vapor enters the interior passages. References70 are apertures in the wall of channel 18 for discharging cesium vaporinto the immediate vicinity of the exposed surfaces of the cathodeelectrodes.

What is claimed is:

1. A magnetohydrodynamic generator comprising:

a. a source of noble gas under pressure b. means for heating the noblegas c. means for providing cesium vapor in the said noble gas d. achannel of refractory insulating material connected to receive the noblegas moving responsively to its pressure, after it has been heated andprovided with cesium vapor, and guide it along a path normal to e. amagnetic field transverse to the said path, and

between f. a plurality of anode electrodes and a plurality of cathodeelectrodes supported to present conductive surfaces to the gas in itsmotion along the path and having electrical connection to the exteriorof the channel g. gas exit means connected to the end of the path in thechannel;

in which h. the surfaces of said cathode electrodes exposed to themoving gas in its motion along the path are of tungsten at a temperatureof at least 1,400" K and immediately adjacent to the said exposedsurfaces the concentration of cesium vapor is at least 10 atoms percubic centimeter; and

i. the molecular concentration of cesium in the flowing gas is between0.1 and 0.3 percent of the total flowing gas.

2. A generator as claimed in claim 1 in which j. the concentration ofcesium in the noble gas as received by the channel is at least 10 atomsper cubic centimeter, and j k. the pressure of the gas in the channel issuch that the concentration of cesium atoms is from 0.1 to 0.3 percentof the total number of gas atoms.

3. A generator as claimed in claim 1 in which l. the concentration ofcesium in the noble gas as received by the channel is less than 10"atoms per cubic centimeter, and

m. cathode cesium means are provided with inject additional cesium vaporimmediately adjacent to the exposed cathode surfaces to provide aconcentration of at least 10" cesium atoms per cubic centimeter, and

n. the concentration of cesium atoms in the gas which enters the gasexit means is not more than 0.3 percent of the total number of gasatoms.

4. A generator as claimed in claim 3 in which 0. the said cathodeelectrodes comprise tungsten provided with interior passages extendingto the surface exposed to the moving gas, and

p. the cathode cesium means comprise a store in which cesium is keptmolten connected to the said interior passages at a part of the tungstennot exposed to the moving gas.

5. A generator as claimed in claim 4 in which q. the interior passagesare sufficiently small to stop the passage of liquid molten cesium andr. the liquid molten cesium is in contact with the tungsten.

6. A generator as claimed in claim 4 in which 5. the molten cesium isapart from the tungsten and its vapor enters the interior passages.

7. A generator as claimed in claim 3 in which t. the cathode cesiummeans comprise a store of cesium and means for vaporizing the cesium,connected to u. apertures m the channel wall ad acent to the exposedsurfaces of the cathode electrodes for discharging the cesium vapor intothe immediate vicinity of the exposed surfaces.

M-m mma) PATENT @FFICE m5 wmmmmm Patent No. 3, 708, 704 Dated January 2,1973 Inventofls) Bert (nmi) Zauderer w I I I 4 H 1' I Column 8, line 4,change "with" to whlch Signed andvsea' led this 17th day of April 197$(SEAL) Attest:

EDWARD M.PLETCHER,JR. i ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

2. A generator as claimed in claim 1 in which j. the concentration ofcesium in the noble gas as received by the channel is at least 1017atoms per cubic centimeter, and k. the pressure of the gas in thechannel is such that the concentration of cesium atoms is from 0.1 to0.3 percent of the total number of gas atoms.
 3. A generator as claimedin claim 1 in which l. the concentration of cesium in the noble gas asreceived by the channel is less than 1017 atoms per cubic centimeter,and m. cathode cesium means are provided with inject additional cesiumvapor immediately adjacent to the exposed cathode surfaces to provide aconcentration of at least 1017 cesium atoms per cubic centimeter, and n.the concentration of cesium atoms in the gas which enters the gas exItmeans is not more than 0.3 percent of the total number of gas atoms. 4.A generator as claimed in claim 3 in which o. the said cathodeelectrodes comprise tungsten provided with interior passages extendingto the surface exposed to the moving gas, and p. the cathode cesiummeans comprise a store in which cesium is kept molten connected to thesaid interior passages at a part of the tungsten not exposed to themoving gas.
 5. A generator as claimed in claim 4 in which q. theinterior passages are sufficiently small to stop the passage of liquidmolten cesium and r. the liquid molten cesium is in contact with thetungsten.
 6. A generator as claimed in claim 4 in which s. the moltencesium is apart from the tungsten and its vapor enters the interiorpassages.
 7. A generator as claimed in claim 3 in which t. the cathodecesium means comprise a store of cesium and means for vaporizing thecesium, connected to u. apertures in the channel wall adjacent to theexposed surfaces of the cathode electrodes for discharging the cesiumvapor into the immediate vicinity of the exposed surfaces.